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I feel like this is less of a big decision and more of a 'duh' sort of situation. To my understanding this isn't saying that all AI art violates copyright, but that AI art which does violate copyright can't be used.
Like if i took a picture of Darth Vader and handed it to NightCafe to fool around with, that still belongs to Disney. Steam is legally required to act if a valid DMCA is sent, and to adhere to the court's ruling in the case of a dispute.
I feel like this is a reassurance that they intend to obey copyright law rather than a restriction of all AI art. Basically they're saying that if you DMCA someone in good faith on the basis of derivative works, they'll play ball.